1. Field of the Invention
The present invention is directed to a method for the presentation of projection images or tomograms from 3D volume data of an examination volume, particularly from MR or CT volume data, wherein a grayscale image of a prescribable projection or of a prescribable section that is composed of individual pixels is calculated from the 3D volume data.
2. Description of the Prior Art
The presentation of projection images or tomograms from 3D volume data plays a significant part in many technical fields, particularly in the field of imaging medical technology. Imaging methods such as magnetic resonance (MR) tomography and computed tomography (CT) acquire information from the entire examination volume of the examined body. X-ray computed tomography is a specific X-ray tomogram exposure method wherein transverse tomograms, i.e. images of body slices oriented essentially perpendicularly to the body axis, are obtained. For this purpose, the examination volume is transirradiated in slices from a number of angles, so that a three-dimensional volume dataset is obtained. The tomograms to be presented are calculated from the 3D volume data by means of suitable projection methods. In addition to such tomograms, other image presentations, for example of surface structures of subjects in the examination volume, can be calculated from the 3D volume data with the assistance of computed tomography techniques.
In magnetic resonance tomography, magnetic resonance signals of an examination volume are acquired using specifically switched, dynamic magnet fields, referred to as gradient fields, and radio-frequency pulses. A magnetic resonance tomogram is calculated from the magnetic resonance signals obtained from different, small body volumes, referred to as voxels, which are being stored as a 3D volume dataset. The position and orientation of the slices of the examination volume to be presented can be arbitrarily selected within broad ranges in both X-ray computed tomography as well as in magnetic resonance tomography. In addition to generating these tomograms or sectional images, different projections from the measured 3D volume data can be calculated and displayed on magnetic resonance tomography. A method that is just being introduced in MR angiography, referred to as MIP (Maximum Intensity Projection) technique, leads to a projection image that enables a presentation of larger vessel sections of the body. In this method, a family of parallel rays is placed through the three-dimensional volume dataset, and only that point having the highest signal intensity is sought along each individual ray. Since blood vessels are imaged with high signal intensity, a respective picture element that belongs to a vessel is thus selected along each ray. This point is then entered into the projection plane, which lies perpendicular to the rays, at the end of the respective ray. A projection image of the vascular system that makes a plastic impression arises in this way. The same technique can also be applied in computer tomography. The tomograms or projection images generated with this method usually are calculated and displayed as grayscale images.
The images of the calculation or projections methods that are utilized for the image presentation usually contain no depth information, so that the spatial relationships of the anatomical details presented in the image relative to one another cannot be clearly perceived and misinterpretations by the viewer are possible. Various projection directions are shown in continuous succession as a moving image for visualizing the spatial relationship of anatomical details, for example in the MIP technique, in order to thereby convey a spatial impression to the viewer. This, however, is computationally complicated and nonetheless cannot always be applied. The presentation is not immediately clear if covering exists, since a “front-back” transposition corresponds to a reversal of the rotational sense if the other presentation parameters are unaltered. Further, special devices such as 3D glasses are known that offer respectively different viewing directions to each eye. These glasses, however, assume a good spatial visualization and have not yet been able to prevail in practice. Back voxels are presented attenuated due to the intensity coding of the distance, so that a loss of contrast occurs.
German OS 44 36 263 discloses a method as well as a system for the presentation of 3D measured data of a volume in two-dimensional tomograms, wherein the tomogram of the slice to be presented is overlaid with the tomograms of neighboring slices. The information important to the viewer, namely which of the image details lie in front of or behind the slice to be presented, is achieved by different chromatic presentation of the overlaid tomograms. The method serves the purpose of conveying the direction of the further course of the subjects visible in the tomogram to the viewer of a tomogram. Although the three-dimensional relationship between the slice to be presented and its immediate environment is visualized for the viewer of a tomogram with this method, the method does not supply any depth information that, in particular, can be important in projection images.